1. Field of the Invention
The present invention relates to a method of controlling power of pump light in wideband Raman amplification using an optical fiber.
2. Description of the Related Art
(Distributed Raman Amplification)
In the field of communication systems using an optical fiber transmission line, development is under way for commercializing distributed Raman amplification (DRA) techniques. Optical fiber for use in basic transmission networks today employs quartz glass as a base material. Raman amplification is a phenomenon that making signal light and pump light having a frequency about 13 THz higher than that of the signal light be simultaneously incident on quartz glass causes part of energy of the pump light to move to the signal light through the stimulated Raman scattering effect of the quartz glass. As a result, the signal light is subjected to amplification. Gain obtained as a result of Raman amplification will be referred to as a Raman gain hereinafter. An actual Raman gain has such wavelength dependency as shown in FIG. 14, which will be referred to as a Raman gain profile hereinafter.
Distributed Raman amplification is a mode of applying pump light to an optical fiber which transits signal light to obtain the Raman amplification effect with the optical fiber transmission line itself as an amplification medium. Since a propagation loss of a transmission line is compensated for by Raman amplification, an optical fiber transmission system using distributed Raman amplification enables extension of a distance in which the signal is transmittable.
Example of an optical fiber transmission system using distributed Raman application is shown in FIG. 18. In each repeater plant, pump light from a pump light source 30 is applied to a transmission line fiber 10 through a WDM coupler 20 to obtain the Raman amplification effect.
(Raman Gain Slope)
A Raman gain (dB), which is generated when pump light of certain power (W) is applied to an optical fiber as a Raman amplification medium, normalized by the power of the pump light is referred to as Raman gain slope (dB/W). Description will be in the following made of that measurement of Raman gain slope is important in distributed Raman amplification.
Raman gain slope varies with an individual fiber. To begin with, optical fibers laid as basic transmission networks have various kinds and Raman gain slope depends on a mode field diameter (core diameter), an amount of GeO2 addition, absorption of water (OH), etc. of the optical fibers. These parameters also vary with a manufacturer, manufacturing time and a lot of an optical fiber.
Another chief factor in variation is a station loss. In a large repeater plant in particular, there exist connector connections at several sites from a room where a pump light source is placed to a transmission line fiber to involve a loss of several dB in many cases. With a transmission system using no distributed Raman amplification, station loss can be taken into consideration as one with a section loss between repeaters. In distributed Raman amplification, however, a loss caused before pump light reaches a transmission line fiber is special and therefore needs another specification.
Thus, when distributed Raman amplification is conducted on an existing transmission line whose parameters affecting a gain largely vary, it is difficult to predict power of pump light required for obtaining a desired Raman gain in advance. Adjustment at site is therefore needed which costs labor and time. Elimination of the need of adjustment could be realized when conditions of a site such as properties of a transmission line optical fiber and loss characteristics in a repeater plant can be measured as Raman gain slope. This enables power of pump light required for obtaining a certain gain to be predicted with high precision.
(Composite Raman Gain Profile)
As shown in FIG. 14, since a band in which a Raman gain profile obtained by a single pump wavelength has a peak is about 15 nm, when a wider band is required, it is necessary to make a plurality of pump lights of different wavelengths simultaneously enter a transmission line and overlap the same to compose a gain profile as shown in FIG. 15. Raman gain profile thus obtained by pump using a plurality of pump wavelengths will be hereinafter referred to as a composite Raman gain profile. On the other hand, a Raman gain profile obtained using a single pump wavelength will be referred to as a single wavelength pump Raman gain profile.
(Conventional Method of Obtaining Desired Composite Raman Gain Profile)
In order to make a desired gain be generated at a desired wavelength band by using a composite Raman gain profile, it is necessary to solve an optimization problem of how much power is to be excited at which wavelength. When optimization is insufficient, irregular gains will be generated in a wavelength band in which a gain profile should be flat, so that a gain profile will incline or useless gain will be generated at an unnecessary wavelength band. In terms of cost reduction, optimization should be conducted so as to have as small the number of pump light sources and total power of pump light as possible. This is because an increase in the number of light sources is followed by not only by an increase in costs of light sources but also by reduction in cost performance caused by an increase in costs of parts necessary for multiplexing, by a power loss at the time of multiplexing, etc.
This problem can be solved by trial and error. In a case of FIG. 15, for example, a composite Raman gain profile which has a flat part of 7.5 dB in a band of about 80 nm is attained in an SMF by five pump lights of different wavelengths. Although such method realized by trial and error is possible in a laboratory, it is impractical to execute the method at a site of installation in terms of labor, time and skill required for adjustment.
(Necessity of Controlling Output Signal Power to be Constant in Amplification Repeater)
Since in a terrestrial transmission system, a repeater installation place is limited as compared with a submarine-based system, it is very unlikely that fiber losses between repeater plants have constant values. In addition, it is known that fibers are liable to be affected by weather and atmospheric phenomena and be increased or decreased in day or seasonal cycle depending on a surrounding temperature of an installation place. Moreover, signal transmission power of an immediately preceding repeater has a little fluctuation or error. As a result, transmission signal power applied to a repeater has a margin.
On the other hand, in an amplification repeater, it is desirable to conduct control to maintain power of a signal having been amplified, that is, relay output power, by increasing or decreasing a gain. The reason is that multi-stage relay with a gain fixed might result in having a signal level going excessively higher or lower than expected. Since control for maintaining power of a signal having been amplified is equivalent to control of canceling a loss including fluctuation in a section preceding to the repeater by means of the repeater to restore the signal power, a signal level in the multi-stage relay system can be stabilized.
Conventional optical amplification repeater is formed of an Er-doped optical fiber amplifier (EDFA) and in general includes a control circuit for maintaining signal output to be constant by adjusting power of pump light to an Er-doped optical fiber. When Raman amplification is to be applied to amplification relay, desired is a mechanism that enables a Raman gain to be dynamically changed to have constant signal output power.
As described in the foregoing, for obtaining a desirable composite Raman gain profile, adjustment of power of pump light by trial and error is conventionally required. On the other hand, because Raman gain slope of a laid optical fiber largely varies with each fiber, optimum power of pump light varies with each fiber accordingly, so that adjustment at site requiring labor and time is required.
Moreover, even if power of pump light is appropriately set at the time of installation, with power of pump light fixed, the power might deviate from an optimum point as a line changes with time. Taken as an extreme example is a case where a fiber between stations disconnects to switch to a spare fiber between stations. Also, even with the same fiber between stations, a loss in the station might change when wiring in the station is modified or an optical connector is attached or detached, thereby causing deviation of optimum power of pump light. In addition, as described above, there occur variation of a loss of a fiber between stations and variation of signal transmission power of an immediately preceding repeater. In order to cope with these time-changing phenomena, required is a mechanism for active control to have optimum power of pump light at any time.
As such a mechanism, proposed is, for example, a method of obtaining a desired Raman gain profile by trial and error by individually changing power of each pump light while monitoring a composite Raman gain profile by means of an optical spectrum analyzer (e.g. Japanese Patent Laying-Open (Kokai) No. 2001-007768). The method employing monitoring of a composite Raman gain profile, however, has a fatal shortcoming that monitoring is impossible unless signal light exists there. Some of customers have a request for reducing the number of initial multi channels in a wavelength multiplex communication system to a minimum necessary number and additionally increasing the number of channels according to an increase in a demand for communication traffic in order to suppress initial investment. In this case, it is common that together with the request, another request is made that operation of adding a channel should be executed without affecting an existing channel while it is operated. For this purpose, a Raman gain of a wavelength band in which channel is yet to be applied should be controlled with sufficient precision and such a manner of conducting negative-feedback control by monitoring a composite Raman gain profile is hard to be applied.
That a composite Raman gain profile can be maintained without monitoring and can actively cope with a time-changing phenomenon is equivalent to that open-loop control of the composite Raman gain profile is possible. None of such a method has been ever proposed.